Transmissive, refractive optical units serve to provide optical images of objects. Light (e.g. visible light or infrared light) propagates through the lenses of the optical unit and, in the process, it is refracted at the interfaces of the lens on account of the difference in the refractive index of the lens material and the refractive index of the surroundings of the respective lens. The deflection of the light is determined by the form of the interface and the difference in the refractive indices at the interface. The interfaces at which light is refractive for optical imaging may also be referred to as optically effective faces.
In the simplest case, transmissive, refractive optical units may comprise only a single lens; however, a plurality of lenses are usually arranged in succession in order to combine the properties thereof. In the case of applications in the range of visible light, the refractive power of a plurality of lenses should often be combined for particularly pronounced magnification or reduction.
Combining a plurality of lenses in succession in the light propagation direction requires a sufficiently accurate radial and axial alignment and a sufficiently accurate alignment in terms of inclination of these lenses or the optical axes thereof with respect to a common optical axis of the lens arrangement.
In order to ensure this, different alignment and fixation variants were developed over time for optical elements (lenses). An overview of these is provided in H. Naumann et al., Handbuch Bauelemente der Optik [Optical component handbook], published by Carl Hanser Verlag, Munich, 7th edition, 2014.
In one variant, which is also realized in US 2016/0282593 A1, an optical element is supported axially in a common casing. All further optical elements in the common casing are supported at the respective preceding optical element by way of spacers, and a fastener engages axially behind the last optical element. Each optical element is individually aligned in the radial direction by the common casing. In this design, possible errors (e.g. different edge thicknesses of the spacers and optical elements) may add up and propagate in the optical assembly such that there may be relatively pronounced oblique positioning of the optical axes of the last optical elements.
A propagation of errors is avoided if each optical element obtains a dedicated stopping face in the common casing and a dedicated fixation. However, this design requires a diameter that increases from optical element to optical element in order to ensure the assemblability.
In both designs, the radial alignment of the optical elements is brought about by the inner wall of the common casing in relation to the radial edge (lateral face) of the respective optical elements. The local distance between the radial edge and the optical axis of an optical element is subject to production tolerances, as a result of which a certain shift of the optical axis of the optical element emerges in relation to the common optical axis.
Moreover, a noticeable radial gap remains between the edge (lateral face) of the optical elements and the inner wall of the common casing, said gap likewise facilitating a shift of the optical elements in relation to the common optical axis and, as a result thereof, generally bringing about optical aberrations. The gap is required to compensate production tolerances of the components, and also to prevent radial clamping of the optical element and a corresponding buildup of tension in the optical element in the case of a relative radial thermal expansion of an optical element in relation to the common casing, which could otherwise lead to a distortion of the optical elements and would have an adverse effect on the imaging properties. If the radial gap is too small, in particular in view of possible temperature variations in the case of storage, transport and normal use, it is even possible for the optical element to be destroyed. Conversely, the decentering, and hence also the optical aberrations, increase in the case of a very large radial gap.
DE 10 2004 048 064 A1, or else DE 10 2005 023 972 A1, has disclosed microscope objective lenses, in which each lens is respectively arranged in a mount (holder) and set radially in a common casing by way of the respective mount.
Each individual lens is fixed in a dedicated mount within the scope of the so-called lathe centering, with the external contour thereof being post-processed on special alignment turning machines. The optical axis of the optical element is measured during this processing. Post-processing of the mount is carried out in such a way that the orientation-determining contours on the post-processed mount are placed more accurately in relation to the optical axis of the optical element. In the case of so-called alignment adhesive bonding, the optical axis of an optical element is likewise measured and the optical element is adhesively bonded in its mount in such a way that said optical element is placed precisely into the mount. As a result of this, it is possible to minimize tolerances in the radial position of the optical axis of the optical element in relation to the mount or in relation to the edge of the mount, which is decisive for the alignment in a common casing.
Lathe centering and alignment adhesive bonding are options for reducing the optical aberrations in an optical system, in particular by minimizing tolerances in the radial distance of the optical axis of an optical element from the lateral face of the mount that is relevant for the radial alignment in the common casing. However, different (radial) thermal expansions of optical element and mount, or of mount and common casing, remain problematic in these designs. Moreover, lathe centering and alignment adhesive bonding are complicated and expensive.